Summary: Biological products such as vaccines, therapeutics and xenotransplants can potentially be contaminated with infectious retrovirus due to induction of an endogenous, latent virus in the cell substrate or xenotranplant tissue or by recombination between endogenous retroviral sequences to produce a novel virus. Additionally, blood and blood products may contain viruses due to the host donor. Thus, to assure public health safety in biolgics, it is critical to develop sensitive retrovirus detection assays and to evaluate potential human risk of retroviruses, which may be of potential safety concern. 1) SFV infections in humans have been reported due to accidental, occupational exposure to infected non-human primates. Therefore, biological products using materials of simian origin need to be demonstrated to be free of SFV. To evaluate the potential risk of SFV infection and transmission in humans, naturally-occurring SFVs were isolated at low passage from peripheral blood mononuclear cells (PBMCs) of rhesus (Rh) and pig-tailed (Pt) macaques. A. In vitro infectivity studies indicated that the naturally-occurring macaque isolates had slower replication kinetics than the laboratory-adapted prototype viruses, such as SFV-1 and SFV-2. In fact, in one human tumor cell line, there was no replication of any of the Rh or Pt isoaltes. Investigations of the viral determinants of latency include the regulatory regions such as the LTR and internal promoter. Additionally, expression and function of the transactivating factor (taf) is also being analyzed. B. To investigate SFV infection due to naturally-occurring viruses in non-human primates and analzye any potential pathogenic effect of virus infection, SFV-negative animals were injected with a virus stock prepared from SFV isolated from a macaque PBMCs. The animals are currently being monitored for virus infection and clinical changes. These results will provide a basis to develop experiments to address potential safety concerns regarding SFV transmission by blood donors. 2) Endogenous retroviruses can potentially contaminate biological products by activation of viral sequences in the cell susbtrate. We have investigated quantitatively the induction of type A and type C retroviruses in a well-characterized mouse cell line using a highly sensitive PCR based reverse transcriptase assay (PERT). The results indicated earlier and more sensitive detection of retrovirus activation as compared to previous traditional assays. Analysis of different chemical inducers indicated different kinetics of endogenous retrovirus activation in mouse cells. Virus induction with different inducers was compared using TEM and RT assays. The results indicated different regulation of endogenous retrovirus production with different inducers and caution in data interpretation due to enhanced RT cellular RT production. To evaluate endogenous retrovirus induction in primate cell substrates used in vaccine production, the cells will be exposed to various chemical inducers, initially, under the conditions that have proven to be successful in the mouse system. The results of these studies will provide strategies to evaluate endogenous retrovirus induction in vaccine cell susbtrates.